A persistent weather pattern is bringing unusually warm, dry, and sunny conditions to much of the western U.S. and southwestern Canada this week. Cities like Los Angeles, Phoenix, and San Francisco may see highs 5–10 degrees above average, while clear nights and light winds create large temperature swings. Stagnant air could trap pollution and form dense, lingering fog, especially in California’s Central Valley. The dry spell may continue until the jet stream shifts south later this week, allowing storms to disperse fog and refresh air quality.